1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to an array substrate having a high storage capacitance and a high aperture ratio, and a fabricating method thereof.
2. Description of the Background Art
A conventional liquid crystal display (LCD) device uses optical anisotropy and polarization properties of liquid crystal molecules. The liquid crystal molecule has a definite orientational order in alignment resulting from its thin and long shape. The alignment direction of the liquid crystal molecule can be controlled by applying an electric field to the liquid crystal molecule. In other words, as the alignment direction of the electric field is changed, the alignment of the liquid crystal molecules also changes. Images are displayed since the incident light is refracted to the orientation of the liquid crystal molecules due to the optical anisotropy of the aligned liquid crystal molecules.
Of the different types of known LCDs, active matrix LCDs (AM-LCDs), which have thin film transistors and pixel electrodes arranged in a matrix form, are the subject of significant research and development because of their high resolution and superiority in displaying moving images.
FIG. 1 is a schematic cross-sectional view partially showing a liquid crystal panel of an LCD device of the background art. In FIG. 1, the liquid crystal panel 20 has an upper substrate 2, referred as a color filter substrate, and a lower substrate 1, referred as an array substrate. The upper and lower substrates 2 and 1 are facing and spaced apart from each other and a liquid crystal layer 10 is interposed therebetween. A black matrix 9 that prevents light leakage and a color filter 8 that selectively transmits light are formed on the inner surface of the upper substrate 4 with an overlapped portion. A common electrode 12 that applies a voltage to the liquid crystal layer 10 is formed on the black matrix 9 and the color filter 8. On the other hand, a pixel electrode 14 that applies a voltage to the liquid crystal layer 10 with the common electrode 12 of the upper substrate 4 is formed at the top of the lower substrate 1. A thin film transistor (TFT) “T” that is a switch of the voltage applied to the pixel electrode 14 is formed on the lower substrate 2.
A storage capacitor “Cst” that keeps the voltage applied to the liquid crystal layer 10 for one frame is formed at a pixel region where the pixel electrode 14 is disposed. The types of the storage capacitor “Cst” can be divided into a previous gate type and a common type. In the previous gate type, the pixel electrode and the previous gate line has an overlapping area and this overlapping area is used as the storage capacitor. In the common type, a common line is formed at the pixel region and the storage capacitor is formed between the common line and the pixel electrode. The previous gate type has advantages in aperture ratio and yield, and the common type has advantages in display quality.
Recently, according to the concentration on LCD devices with high definition and high display quality, storage capacitor of a complex type of the previous gate and the common types is being researched.
FIG. 2 is a schematic plan view partially showing an array substrate of an LCD device of the background art having a common type storage capacitor. In FIG. 2, a gate line 26 having a gate electrode 22 is formed along a row direction and a common line 24 parallel to the gate line 26 is spaced apart from the gate line 26. A semiconductor layer 30 is formed on the gate electrode 22. A source electrode 32 and a drain electrode 34 overlapping the semiconductor layer 30 are spaced apart from each other. A data line 36 connected to the source electrode 32 is formed along a column direction and crossing the gate and common lines 26 and 24. A pixel electrode 46 is formed at a pixel region defined by the gate and data lines 26 and 36 and a storage electrode 38 is formed in the pixel region.
The storage electrode 38 is made of the same material as the data line 36 and is disposed over the common line 24 with a first area. A TFT includes the gate electrode 22, the semiconductor layer 30 and the source and drain electrodes 32 and 34. The pixel electrode 46 is connected to the drain electrode 34 through a first contact hole 42 and connected to the storage electrode 38 through a second contact hole 44. A gate insulating layer is interposed between the common line and the storage electrode 24 and 38 and a passivation layer is interposed between the storage and the pixel electrodes 38 and 46. The passivation layer includes first and second contact holes 42 and 44, and protects the TFT from exterior damage.
In the above-mentioned structure, a storage capacitor “Cst” is formed between the common line 24 and the storage electrode 38 and between the common line 24 and the pixel electrode 46. The capacitance is defined by the following relationship:C=∈A/d 
where C is capacitance, ∈ is permittivity of the interposed dielectric material, A is area of the electrode of the capacitor and d is distance between the electrodes of the capacitor.
Since the gate insulating layer is thinner than the passivation layer, the capacitance between the common line 24 and the storage electrode 38 is larger than that between the common line 24 and the pixel electrode 46. Therefore, the storage capacitance is increased by adding the storage electrode 38. However, since the storage electrode 38 is made of opaque metallic material, which is the same material of the data line 36, an aperture ratio is reduced by adding the opaque storage electrode 38.
FIGS. 3 to 12 are schematic plan views and their schematic cross-sectional views showing forming process of an array substrate for an LCD device of the background art. In these processes, deposition, photolithography and etching are repeated to form the array substrate. FIG. 3 is a schematic plan view showing a forming process of an array substrate for an LCD device of the background art. FIG. 4 is a schematic cross-sectional view showing a forming process of an array substrate for an LCD device of the background art. In FIG. 3 and FIG. 4, a gate line 26 having a gate electrode 22 and a common line 24 are formed on a substrate 1 along a row direction. The common line 24 parallel to the gate line 26 is spaced apart from the gate line 26. A double metallic layer including aluminum (Al) is mainly used as the gate and common lines 26 and 24.
FIG. 5 is a schematic plan view showing a forming process of an array substrate for an LCD device of the background art. FIG. 6 is a schematic cross-sectional view showing a forming process of an array substrate for an LCD device of the background art. In FIG. 5 and FIG. 6, after a gate insulating layer 28 is formed on the entire surface of the substrate 1 having the gate and common lines 26 and 24, an active layer 30a of amorphous silicon (a-Si) and an ohmic contact layer 30b of doped amorphous silicon (doped a-Si) are subsequently formed on the gate insulating layer 28 over the gate electrode 22 to form a semiconductor layer 30. The ohmic contact layer 30b can reduce the contact resistance between the active layer 30a and a following metal layer since an ionic doping process increases the carrier mobility of the ohmic contact layer 30b. 
FIG. 7 is a schematic plan view showing a forming process of an array substrate for an LCD device of the background art. FIG. 8 is a schematic plan cross-sectional view showing a forming process of an array substrate for an LCD device of the background art. In FIG. 7 and FIG. 8, a data line 36 having a source electrode 32, a drain electrode 34 spaced apart from the source electrode 32 and a storage electrode 38 are formed on the substrate having the semiconductor layer 30. The data line 36 crossing the gate and common lines 26 and 24 is disposed along a column direction. The storage electrode 38 overlapping the common line 24 is disposed in the pixel region. Chemical-resistant metal such as molybdenum (Mo) is mainly used as the data line 36, the drain electrode 34 or the storage electrode 38. In this forming process of the source and drain electrodes 32 and 34, the ohmic contact layer 31 between the source and drain electrodes 32 and 34 is eliminated so that a channel ch can be formed by exposing the active layer 30a. 
FIG. 9 is a schematic plan view showing a forming process of an array substrate for an LCD device of the background art. FIG. 10 is a schematic cross-sectional view showing a forming process of an array substrate for an LCD device of the background art. In FIG. 9 and FIG. 10, a passivation layer 40 having a first contact hole 42 and a second contact hole 44 is formed on the entire surface of the substrate. The first and second contact holes 42 and 44 expose the drain and storage electrodes 34 and 38, respectively.
FIG. 11 is a schematic plan cross-sectional view showing a forming process of an array substrate for an LCD device of the background art. FIG. 12 is a schematic view showing a forming process of an array substrate for an LCD device of the background art. In FIG. 11 and FIG. 12, a pixel electrode 46 is formed on the passivation layer 40 and connected to the drain and storage electrodes 34 and 38 through the first and second contact holes 42 and 44, respectively. The pixel electrode 46 is made of a transparent conductive material such as indium tin oxide (ITO) or indium zinc oxide (IZO).
The storage electrode 38 connected to the pixel electrode 46 through the second contact hole 44 forms a storage capacitor “Cst” with the common line 24. Since the distance between the common line 24 and the storage electrode 38 is longer than that between the common line 24 and the pixel electrode 46, the storage capacitance can be increased. However, since the region “A” of the storage electrode 38 of opaque metal also reduces the aperture ratio, it becomes difficult to increase the aperture ratio and the storage capacitance.